(1) Field of the Invention
This invention relates to a means of providing controlled release of bioactive materials. More particularly, this invention relates to the preparation of alginate gel beads that contain a bioactive material dispersed therein.
(2) Description of the Prior Art
U.S. Pat. No. 4,053,627 describes the use of alginate gel discs to release juvenile hormone into an aqueous environment and U.S. Pat. No. 2,441,729 teaches the use of alginate gels as insecticidal jellies. These processes require that the salt which yields the gellant cation, such as calcium, be insoluble or almost insoluble in water, and be admixed with the alginate before adding to water. The salt must not, of itself, yield sufficient cations to gelatinize the alginic component quickly. It is a disadvantage of these processes that gel formation is relatively slow requiring about 10 minutes to 2 hours, during which time the mixture must be kept in a mold to hold the desired shape until gelation occurs.
U.S. Pat. No. 3,328,256 teaches the use of spherical beads for the controlled release of medicaments for use in the medical field. There are several disadvantages with the teachings of this particular process. In the preparation of the spherical beads, the patent speaks of using organic solvents which may be flammable or toxic and the products are entirely foreign to any use in agriculture.
U.S. Pat. No. 3,376,127 teaches the use of a herbicide which is applied to a carrier that is lighter than water as a means for attacking floating plants. No mention of controlled release is made; furthermore, any teaching for the use of floating means is accomplished by addition of solid lighter-than-water materials, or by using organic emulsions.
U.S. Pat. No. 4,107,292 teaches the use of xanthan gum added to formulations containing already existing polymer beads that contain insecticide in order to produce a better suspension of the polymer beads in water. The teaching is solely one of suspending an existing material in a liquid, to avoid settling in the container before spray application.
U.S. Pat. No. 3,649,239 and British Pat. No. 1,271,575 describe fertilizer compositions in alginate carriers that are applied to soil as liquid if the soil is acidic, and as gels if the soil is highly alkaline. In the latter case the gels are made by incorporating an acid gellant as an ingredient in the alginate fertilizer solution. It is a disadvantage that metal ions such as calcium are used only in certain circumstances as a soil application after the fertilizer composition has been applied to the soil.
British Pat. No. 1,236,885 teaches that alginates are useful in preparing multiwall capsules. A serious disadvantage in this process is that multiple process repetitions are required to achieve the necessary result. Furthermore, there is no teaching as to controlled release.